This application relates to a parallel flow heat exchanger, wherein parallel tubes are configured and mounted in a manifold in a manner that minimizes brazing material blocking channels in the tubes.
Refrigerant systems utilize a refrigerant to condition a secondary fluid, such as air, delivered to a climate controlled space. In a basic refrigerant system, the refrigerant is compressed in a compressor, and flows downstream to a heat exchanger (a condenser for subcritical applications and a gas cooler for transcritical applications), where heat is typically rejected from the refrigerant to ambient environment, during heat transfer interaction with this ambient environment. Then refrigerant flows through an expansion device, where it is expanded to a lower pressure and temperature, and to an evaporator, where during heat transfer interaction with another secondary fluid (e.g., indoor air), the refrigerant is evaporated and typically superheated, while cooling and often dehumidifying this secondary fluid.
In recent years, much interest and design effort has been focused on the efficient operation of the heat exchangers (e.g., condensers, gas coolers and evaporators) in the refrigerant systems. One relatively recent advancement in the heat exchanger technology is the development and application of parallel flow, or so-called microchannel or minichannel, heat exchangers (these two terms will be used interchangeably throughout the text), as the condensers and evaporators.
These heat exchangers are provided with a plurality of parallel heat transfer tubes, typically of a non-round shape, among which refrigerant is distributed and flown in a parallel manner. The heat transfer tubes are orientated generally substantially perpendicular to a refrigerant flow direction in the inlet, intermediate and outlet manifolds that are in flow communication with the heat transfer tubes. The primary reasons for the employment of the parallel flow heat exchangers, which usually have aluminum furnace-brazed construction, are related to their superior performance, high degree of compactness, structural rigidity and enhanced resistance to corrosion.
In many cases, these heat exchangers are designed for a multi-pass configuration, typically with a plurality of parallel heat transfer tubes within each refrigerant pass, in order to obtain superior performance by balancing and optimizing heat transfer and pressure drop characteristics. In such designs, the refrigerant that enters an inlet manifold (or so-called inlet header) travels through a first multi-tube pass across a width of the heat exchanger to an opposed, typically intermediate, manifold. The refrigerant collected in a first intermediate manifold reverses its direction, is distributed among the heat transfer tubes in the second pass and flows to a second intermediate manifold. This flow pattern can be repeated for a number of times, to achieve optimum heat exchanger performance, until the refrigerant reaches an outlet manifold (or so-called outlet header). Obviously, in a single-pass configuration, the refrigerant travels only once across the heat exchanger core from the inlet manifold to the outlet manifold. Typically, the individual manifolds are of a cylindrical shape (although other shapes are also known in the art) and are represented by different chambers separated by partitions within the same manifold construction assembly.
Heat transfer corrugated and typically louvered fins are placed between the heat transfer tubes for outside heat transfer enhancement and construction rigidity. These fins are typically attached to the heat transfer tubes during a furnace braze operation. Furthermore, each heat transfer tube preferably contains a plurality of relatively small parallel channels for in-tube heat transfer augmentation and structural rigidity.
In the prior art, the openings to receive the multi-channel tubes are formed in a manifold wall by punching the wall inwardly. The heat transfer tubes are inserted into these openings, but do not extend much further into the manifold past the ends of the punched material, since it would create additional impedance for the refrigerant flow within the manifold, promote refrigerant maldistribution and degrade heat exchanger performance. Since the heat transfer tube edges are located at approximately the same positions as the ends of the punched material of the manifold openings, brazing material has a high potential of flowing into some of the channels during the brazing process and blocking these channels. This is, of course, undesirable and should be avoided, since at least partially blocked heat transfer tubes are not utilized to their full heat transfer potential, have additional hydraulic resistance on the refrigerant side and promote refrigerant maldistribution conditions. All these factors negatively impact heat exchanger performance.